General automatic transmissions mounted on a vehicle include a transmission gear mechanism, and a frictional engagement device which achieves a plurality of gear positions by switching a drive force transmission path of the transmission gear mechanism. The frictional engagement device includes clutches which engage and disengage two rotational members, and brakes which fix the rotation of one rotational member. For example, JP2010-071408A discloses such an automatic transmission.
The clutches disclosed in JP2010-071408A are multi-plate clutches having a plurality of friction plates. Each clutch includes, as rotational members, a drum member engaged with outer circumferences of some of the friction plates by a spline to be axially movable and coupled to a given rotational element of the transmission, and a hub member engaged with inner circumferences of the other friction plates by a spline to be axially movable and coupled to another given rotational element of the transmission.
Further, the brakes are generally multi-plate brakes having a plurality of friction plates similarly to the clutch described above. Each brake is engaged with outer circumferences of some of the friction plates by a spline to be axially movable with respect to a transmission case and includes, as a rotational member, a hub member engaged with inner circumferences of the other friction plates by a spline to be axially movable and coupled to a given rotational element of the transmission.
Note that the drum member of the clutch, the hub member of the clutch, or the hub member of the brake (hereinafter, simply referred to as “the drum member” or “the hub member”) is provided with a drive force transmitting part to which a counter rotational member is coupled to exchange a drive force of an input shaft for receiving an input from a drive source, a gear of a transmission gear mechanism, etc.
Meanwhile, it is recently required for vehicles to weigh less so as to improve fuel efficiency of an engine and save energy, for example, by reducing power consumption when using a motor as a drive source. One discussed approach for this is reducing the weight of an automatic transmission. It may be considered to use aluminum for the drum member and the hub member that have relatively low strength requirements since they are heavy due to having large diameters and also tangential loads are comparatively low among rotational elements constituting the automatic transmission.
On the other hand, the drive force transmitting part that exchanges the drive force with the counter rotational member in the drum member or the hub member is, in many cases, provided at a rotation axis side of the transmission, and therefore the tangential load is larger than that of an outer circumferential part where the friction plates are fitted. Thus, the strength of aluminum is considered to be insufficient for the drive force transmitting part.
In this regard, it may be considered to form the drive force transmitting part with a ferrous metal. However, in this case, different materials, that is the ferrous metal forming the drive force transmitting part of the drum member or the hub member and aluminum forming the other parts, need to be joined by welding or the like, and the joining strength at the joint portion may become insufficient. It may also be considered to couple them with a bolt, but this will cause an increase in the number of parts, in size, etc.
Further, the plurality of friction plates engaged with the spline part of the drum member or the hub member have small axial dimensions (thicknesses). Therefore, a surface pressure on the spline part at positions where the spline teeth engage with the friction plates becomes high and the strength of the engaged portions of the spline part may become insufficient. In this regard, it may be considered to reduce the surface pressure against the drum member or the hub member by increasing the axial dimensions of the friction plates, but this will cause an increase in the axial dimension of the clutch or the brake, resulting in a size increase of the automatic transmission.
Meanwhile JP2016-101606A discloses an art for integrating aluminum and ferrous metal by electric resistance welding in which the ferrous metal and aluminum containing a given amount of silicon are pressure-welded and then power is applied. It may be considered to apply this joining technique to the drum member and the hub member.
However, the insufficient joining strength still cannot be avoided with such a joining technique.